I think it would be awful if they removed the Princesses from the PP just for Anna and Elsa (yes, I did say "just"). They waited ages to get a decent meet and greet :-/
Here's a bounce around idea:
How about the old Woody's round up area for Christmas? They already do a makeover for Santa, and the farm area is disused too - a whole Winter Wonderland for them (and Santa) would be lovely! I know Fantasyland is a better place though, but how about transforming the Castle stage like the old Christmas meet and greets with the Princesses?? Putting them in the pavillion will mean queing twice as no doubt they will be seperate and not together :-/
If there is to be a meet and greet (remember most friends of Princesses are versitile and have many talents ;-) ) then thet better have been thought out well!!